1. Technical Field
This invention relates generally to thermal printers and, more particularly, relates to a method for minimizing curling of transparent media during printing.
2. Background Art
To produce a high quality continuous tone print using the thermal printing process, a dye donor sheet in dye transferring contact with a dye receiver sheet is passed through the nip formed between a thermal print head and the transfer print drum. Dye is transferred from the dye donor into the receiver layer that may be coated up on either a reflective support, if it is desired to directly view the images, or upon a transparent support if the image is to be projected. The reflective receiver sheet may be comprised of any opaque substrate sheet such as paper, synthetic paper, or resin coated paper with a dyeable polymer or resin coated upon one surface. The transparent receiver sheet may be comprised of any light transmissive substrate such as polyester film coated with a dye receiving layer upon one surface. When image formation is effected using these receiver sheets, warping of the receiver sheet occurs.
This warping is referred to as curl and is the result of heating the surface of the receiver sheet nearer the thermal head during dye transfer. The curl problem becomes severe as attempts are made to print transparencies with high optical image densities (D.sub.max &gt;1.8) which require the support surface nearer the print head to experience temperatures significantly higher than the glass transition temperatures of most common transparent support materials employed in transparent receiver sheets. This can cause problems with the receiver sheet transport and registration when printing color images in a thermal printer. In addition, the curl of a transparent receiver affects the quality of the projected image which may be undesirable to the viewer. The preferred situation is to have the finished transparency as flat as possible when placed upon a flat surface.
Approaches to controlling curl have focused on the receiver. For example, there have been synthetic paper sheets of at least three plies, each having different Cobb sizing degrees or internal bond strength to prevent curling when used for facsimile, thermal printing, and others. Another is a paper support containing pigment and a rubbery polymer latex material providing a material that would yield reduced curl when imaged with a thermal head or heat pin. Still another curl prevention layer is known that is coated upon either surface adjacent to the substrate and is comprised of a non-heat expandable/contractible resin, preferably an acrylic resin. Addition of this layer is believed to prevent curl when the substrate is used for thermal printing. Each of these approaches, however, add additional expense to the cost of manufacturing the media because of the addition of materials and/or steps required to produce the support. Those that add materials that increase opacity are not useful in producing transparent receiver sheets. It will, therefore, be appreciated that it is highly desirable to have a transparent media that resist curling.
U.S. Pat. No. 4,892,994, which issued Jan. 9, 1990, to Masaru Tsuchiya et al., for "Curling Prevention Device of Thermal Developing Machine", attacks the curling problem after the receiver emerges from the printing zone. The curling prevention device is provided at the outlet of a thermal developing and/or transfer step and thereafter with a correcting guide passage having a bend in the opposite direction to the direction of the curling appearing in the step. The device end is adapted to permit a thermal developing light sensitive material to pass with a temperature of 50.degree. C. at least the inlet portion of the correcting guide passage. While Tsuchiya et al. apparently minimize curling in the finished print, curling does occur, which must be removed before the final print is delivered. Accordingly, it will be appreciated that it would be highly desirable to have a method for producing transparencies that prevent the formation of curl.
U.S Pat. No. 4,505,603, which issued Mar. 19, 1985, to Masasumi Yana, for "Thermal Transfer Color Printer and a Method Relating Thereto" discloses a thermal printer and illustrates the circuitous path that the receiver media traverses during the printing process. The media traverses a portion of the print drum where the donor is brought into contact with it and the dye is transferred from the donor to the receiver under the influence of the thermal print head. Where paper media is used, curling is not a problem. Where curling is a problem, however, is with transparent media.